Estrogen Increases Apolipoprotein (Apo) A-I Secretion in Hep G2 Cells by Modulating Transcription of the Apo A-I Gene Promoter
This article requires a subscription to view the full text. If you have a subscription you may use the login form below to view the article. Access to this article can also be purchased.
Abstract—Estrogen administration to postmenopausal women has been shown to increase plasma levels of apolipoprotein (apo) A-I. A human hepatoma cell line, Hep G2, was used to test the hypothesis that estrogen increases the hepatic production of apo A-I by modulating gene expression. When Hep G2 cells were treated for 24 hours with E2, the apo A-I content in the medium increased 4.3±1.0-fold at 10 μmol/L E2 and 1.8±0.4-fold at 1 μmol/L E2 compared with untreated cells. A time-course experiment indicated that there was no E2-dependent (10 μmol/L) increase in apo A-I medium content at 1 hour and 2 hours and that apo A-I was 165% of controls at 6 hours and 440% at 24 hours. Hep G2 cells were transfected, by the cationic lipid method, with constructs containing serial deletions of the 5′ region of the apo A-I gene (−41/+397, −256/+397, and −2500/+397) cloned in front of the luciferase gene and with or without a 7-kb region spanning the apo C-III/A-IV intergenic region, which has been shown to contain regulatory elements for the expression of the apo A-I gene. With the exception of the construct containing only the basal promoter (−41/+397), the expression of all constructs was 2- to 3-fold greater in the presence of E2. The smallest construct that maintained E2 responsiveness, the −256/+397 construct, does not contain a typical estrogen-responsive element. In the same transfection experiments, the 4-fold increase in apo A-I in the culture medium was preserved. However, when the same set of transfections was performed by the calcium phosphate precipitation method, the E2 effect on the apo A-I content in the culture medium and on transcription activation was nearly abolished. This effect was probably mediated by Ca2+, because incubation of cells with 20 mmol/L CaCl2 abolished the E2 response. In conclusion, E2 increases apo A-I production in hepatic cells by increasing the transcription of the apo A-I gene.
- Received September 24, 1998.
- Accepted July 8, 1999.